Recently, new flat panel display technologies replacing cathode ray tubes are adopted in portable communication apparatuses, notebooks, and mid or large terminals. Along those apparatuses, LCDs (liquid crystal displays) are mostly used for monitors of the mid or large terminals such as notebooks. LCD monitors are competitive in terms of resolution, color display, image quality, and low power consumption, compared to other display devices. Meanwhile, organic electroluminescence devices (hereinafter, referred to as “organic EL”) exhibit superior characteristics in brightness, angle of view, response speed, and power consumption, compared to TFT-LCDs. In particular, the fast response speed of the organic EL than other display devices is suitable for IMT-2000 mobile phones in which motion picture is necessary. Since the organic EL uses organic compounds having a superior fluorescence or phosphorescence efficiency, it is easy to develop a substance having various band gaps through design and synthesis of molecules. Also, since it has a low manufacturing temperature, the organic EL can be manufactured on not only glass but also a plastic substrate.
In order to display image data on a display panel to which a variety of technologies are applied, pixels arranged in vertical and horizontal directions of the display panel are driven by using image data. In the panel, the horizontal length is usually greater than the vertical length so that a user can feel comfortable. However, in some cases such as a display window of a mobile phone, the vertical length is greater than the horizontal length. A circuit driving the pixels of a panel in the horizontal direction is characteristically referred to as a gate driving circuit since the circuit is usually connected to gates of transistors constituting the pixels. In contrast, a circuit driving the pixels in the vertical direction is referred to as a source driving circuit since the circuit is usually connected to sources of pixel transistors. A portion driven by the gate driving circuit is usually used to select a pixel array in the horizontal direction while the source driving circuit is usually used to access data of a selected pixel.
FIG. 1 is a block diagram of a typical display panel driving system. Referring to FIG. 1, the operation of a typical display panel driving system 100 is described.
Image data to be transferred to a display panel 110 is stored in a memory cell array portion 140 in the form of binary information. The data is written in the memory cell array portion 140 through a logic control portion 170 and a memory glue logic 180 from the outside of a driving system shown in FIG. 1. The data read from the memory cell array portion 140 is transferred to a source data buffer 130 and temporarily stored therein and then transferred to the source driving circuit 120. A gate driving circuit 150, a gate control circuit 160 and the data transferred to the source driving circuit 120 finally drive pixels of the display panels 110 so that a user can view a desired image. This conventional display driving system is generally constituted by using several semiconductor IC chips. However, with a recent trend that more number of functions are incorporated into a signal chip, the number of IC chips constituting the system is decreasing.
The way in which data is transferred from the memory cell array portion 140 to the source data buffer 130 is different from that of typical memory products, for example, a standard DRAM or SRAM. In the standard DRAM or SRAM, only the number of columns required for a single cycle are selected by a column address via a column selection gate connected to one end of a bit line, and data is input and output through the gate. In the display memory product like the present invention, however, data of the entire columns connected to the memory cell array portion 140 are simultaneously selected for a single operation cycle through another additional selection gates. The selected data is temporarily stored in the source data buffer 130. At this time, since the source data buffer 130 made of hundreds through thousands of repetitious circuits are all enabled to operate, a large amount of current is consumed.
When current is consumed for such a short time in a semiconductor chip, many unexpected effects occur. A typical unexpected effect is a ground bounce phenomenon generated due to a peak current which makes operation of a circuit unstable and delays operation time. As operation current flowing for a short time increases, that is, the current at a peak value increases, a voltage of a ground node increases proportionally. Since the above phenomenon is well know, a detailed description thereof will be omitted.
One of the conventional methods to solve the above peak current problem is to reduce a switching speed of an output circuit in a semiconductor memory product. The method is disclosed in an article entitled “A 21 ns 32 k×8 CMOS Static RAM with a Selectively Pumped P-well Array” (IEEE, J. of Solid-State Circuits, vol. SC-22 No. 5 Oct. 1987).
Also, a technology which enables a stable operation at a high frequency by providing a data bus sensing portion that is enabled with a delay time adjusted to vary by an operation mode according to a CAS latency information value in SDRAM, is disclosed in Korean Patent Publication No. 2001-0047531 (15 Jun. 2001).
However, in the conventional technologies, since data of the entire columns are not simultaneously accessed, the problem due to the peak current is not quite serious. In contrast, in the circuit exclusively designed for a display use, since data of the entire columns of the memory cell array portion are simultaneously transferred to the source data buffer, the problem due to the peak current becomes more serious.